Topochemical Deintercalation of Al from MoAlB: Stepwise Etching Pathway, Layered Intergrowth Structures, and Two-Dimensional MBene

The synthesis of refractory materials usually relies on high-temperature conditions to drive diffusion-limited solid-state reactions. These reactions result in thermodynamically stable products that are rarely amenable to low-temperature topochemical transformations that postsynthetically modify subtle structural features. Here, we show that topochemical deintercalation of Al from MoAlB single crystals, achieved by room-temperature reaction with NaOH, occurs in a stepwise manner to produce several metastable Mo–Al–B intergrowth phases and a two-dimensional MoB (MBene) monolayer, which is a boride analogue to graphene-like MXene carbides and nitrides. A high-resolution microscopic investigation reveals that stacking faults form in MoAlB as Al is deintercalated and that the stacking fault density increases as more Al is removed. Within nanoscale regions containing high densities of stacking faults, four previously unreported Mo–Al–B (MAB) intergrowth phases were identified, including Mo₂AlB₂, Mo₃Al₂B₃, Mo₄Al₃B₄, and Mo₆Al₅B₆. One of these deintercalation products, Mo₂AlB₂, is identified as the likely MAB-phase precursor that is needed to achieve a high-yield synthesis of two-dimensional MoB, a highly targeted two-dimensional MBene. Microscopic evidence of an isolated MoB monolayer is shown, demonstrating the feasibility of using room-temperature metastable-phase engineering and deintercalation to access two-dimensional MBenes

Very-Large-Scale Integrated High-QQ Nanoantenna Pixels (VINPix)

Metasurfaces provide a versatile and compact approach to free-space optical manipulation and wavefront shaping. Comprised of arrays of judiciously-arranged dipolar resonators, metasurfaces precisely control the amplitude, polarization, and phase of light, with applications spanning imaging, sensing, modulation, and computing. Three crucial performance metrics of metasurfaces and their constituent resonators are the quality factor ($Q$-factor), mode-volume ($V_m$), and the ability to control far-field radiation. Often, resonators face a trade-off between these parameters: a reduction in $V_m$ leads to an equivalent reduction in $Q$, albeit with more control over radiation. Here, we demonstrate that this perceived compromise is not inevitable $-$ high-$Q$, subwavelength $V_m$, and controlled dipole-like radiation can be achieved, simultaneously. We design high-$Q$, very-large-scale integrated silicon nanoantenna pixels $-$ VINPix $-$ that combine guided mode resonance waveguides with photonic crystal cavities. With optimized nanoantennas, we achieve $Q$-factors exceeding 1500 with $V_m$ less than 0.1 $(\lambda/n_{air})^3$. Each nanoantenna is individually addressable by free-space light, and exhibits dipole-like scattering to the far-field. Resonator densities exceeding a million nanoantennas per $cm^2$ can be achieved. As a proof-of-concept application, we demonstrate spectrometer-free, spatially localized, refractive-index sensing utilizing VINPix metasurfaces. Our platform provides a foundation for compact, densely multiplexed devices such as spatial light modulators, computational spectrometers, and in-situ environmental sensors

Confined Chemical Fluid Deposition of Ferromagnetic Metalattices

A magnetic, metallic inverse opal fabricated by infiltration into a silica nanosphere template assembled from spheres with diameters less than 100 nm is an archetypal example of a “metalattice”. In traditional quantum confined structures such as dots, wires, and thin films, the physical dynamics in the free dimensions is typically largely decoupled from the behavior in the confining directions. In a metalattice, the confined and extended degrees of freedom cannot be separated. Modeling predicts that magnetic metalattices should exhibit multiple topologically distinct magnetic phases separated by sharp transitions in their hysteresis curves as their spatial dimensions become comparable to and smaller than the magnetic exchange length, potentially enabling an interesting class of “spin-engineered” magnetic materials. The challenge to synthesizing magnetic inverse opal metalattices from templates assembled from sub-100 nm spheres is in infiltrating the nanoscale, tortuous voids between the nanospheres void-free with a suitable magnetic material. Chemical fluid deposition from supercritical carbon dioxide could be a viable approach to void-free infiltration of magnetic metals in view of the ability of supercritical fluids to penetrate small void spaces. However, we find that conventional chemical fluid deposition of the magnetic late transition metal nickel into sub-100 nm silica sphere templates in conventional macroscale reactors produces a film on top of the template that appears to largely block infiltration. Other deposition approaches also face difficulties in void-free infiltration into such small nanoscale templates or require conducting substrates that may interfere with properties measurements. Here we report that introduction of “spatial confinement” into the chemical fluid reactor allows for fabrication of nearly void-free nickel metalattices by infiltration into templates with sphere sizes from 14 to 100 nm. Magnetic measurements suggest that these nickel metalattices behave as interconnected systems rather than as isolated superparamagnetic systems coupled solely by dipolar interactions